(1) Field of the Invention
The present invention relates to a soundproofing covering having resonators, to a panel including the covering, and to an aircraft.
The invention thus lies in the field of treating acoustic nuisance on board an aircraft.
(2) Description of Related Art
The present invention relates more particularly to systems for treating noise. Reducing the sound level of noise is an ever-increasing problem given the impacts of noise on the comfort and the health of people. This problem is encountered particularly, but not exclusively, in the technical field of aircraft, and in particular of rotary wing aircraft.
A rotary wing aircraft has at least one lift rotor that is driven in rotation by a power train, and possibly also a tail rotor for providing the aircraft with yaw control. The power train may include at least one engine and at least one main gearbox (MGB) interposed between the engine and the lift rotor.
Furthermore, an aircraft may have moving mechanical members for cooling equipment on board the aircraft, and in particular for cooling the main gearbox and also electronic equipment. Moving mechanical members may also be used for delivering air into a cabin. Conventionally, a fan is used for cooling equipment and/or for moving a mass of air.
Thus, such an aircraft generally has multiple noise sources, including for example a main gearbox, turboshaft engine blades, or indeed fans.
Under such circumstances, these sources of noise may immerse a rotorcraft with the entire frequency spectrum that is audible to humans, this audible noise frequency spectrum extending from a frequency component of about 20 hertz (Hz) to a frequency component of about 20,000 Hz.
A structural panel or cladding for an aircraft generally includes at least one wall made of metal, of composite, and/or of thermoplastic materials.
Several solutions have already been proposed in order to attenuate the noise transmitted through such panels.
In order to improve the soundproofing of panels for noise emitted at medium frequencies, conventionally understood as frequencies in the range 300 Hz to 1000 Hz, a first solution in widespread use relies on adding a layer of elastomer material to an outer surface of a panel.
That addition also presents the characteristic of increasing the mass per unit area of the panel. Unfortunately, the soundproofing capacity of a panel mainly follows a “weight” relationship whereby the soundproofing capacity of a panel as measured in decibels varies in proportion to the logarithm of the weight per unit area of the panel. Increasing the weight per unit area of the panel thus tends to optimize the acoustic performance of the panel, but to the detriment of the overall weight of the aircraft.
The elastomer layer also increases the damping of the panel thus making it possible to reduce the negative effect of potential vibratory resonances with auxiliary elements.
However, that first solution suffers from the drawback of increasing the weight of the panel.
Document FR 2 815 603 suggests incorporating a solid body in composite panels of sandwich structure, the solid body being incorporated in a closed space inside the sandwich structure.
That second solution is remarkable in that it enables the sandwich structure to be damped while adding little extra weight.
It is particularly effective for noise emitted in the high frequency range, but its effectiveness is less good for noise emitted at medium frequencies.
Document US 2011/0020592 proposes incorporating dissipative elements, in particular elements of elliptical geometry, in order to improve the damping of structural panels or cladding with little added weight. Such a dissipative element may amplify the movement of masses within a plane that is substantially parallel to the panel that is to be treated.
Under such circumstances, the weight per unit area of a panel may be increased at reduced “weight” cost.
Document FR 2 939 406 suggests interposing a foam between two walls. When a first wall is excited by noise, the second wall provides a vibratory response that seeks to eliminate the original excitation, like a resonator.
The foam optionally includes inserts to avoid sagging.
The effect of those solutions is particularly effective for seeking noise emitted at high frequency. Nevertheless, treating noise emitted at medium frequency is found to be difficult, and may lead to providing a device of high weight.
Active techniques based on actuators, e.g. piezoelectric actuators, have also been proposed, but they can present drawbacks such as complexity, cost, or indeed requiring the incorporation of high-power electronic equipment.
Other concepts are used for improving the soundproofing of panels, e.g. based on adding microperforations in a wall.
Also known are Documents US 2011/0189440, US 2011/0005165, WO 2012/131011, and US 2009/078519.